Implement isAssignableFrom in JS instead of exposing getFakeInstance.

Creating a fake instance and using it as an argument of
`isInstance` is a huge hack. It's OK to do it internally in
`scalajsenv.js`, as long as fake instances do not leak. But
exposing fake instances to the Scala.js code is extremely
dangerous, as it means leaking implementation-specific details at
the IR level, which could be (mis-)processed by the optimizer.

Since `getFakeInstance` was used only for `isAssignableFrom`
anyway, it is much cleaner to directly implement `isAssignableFrom`
in `scalajsenv.js`, and not expose `getFakeInstance` at all.

Author: sjrd

javalanglib/src/main/scala/java/lang/Class.scala

@@ -11,15 +11,16 @@ private trait ScalaJSClassData[A] extends js.Object {
   val isRawJSType: scala.Boolean = js.native
 
   def isInstance(obj: Object): scala.Boolean = js.native
-  def getFakeInstance(): Object = js.native
+  def isAssignableFrom(that: ScalaJSClassData[_]): scala.Boolean = js.native
 
   def getSuperclass(): Class[_ >: A] = js.native
   def getComponentType(): Class[_] = js.native
 
   def newArrayOfThisClass(dimensions: js.Array[Int]): AnyRef = js.native
 }
 
-final class Class[A] private (data: ScalaJSClassData[A]) extends Object {
+final class Class[A] private (private val data: ScalaJSClassData[A])
+    extends Object {
 
   override def toString(): String = {
     (if (isInterface()) "interface " else
@@ -30,11 +31,7 @@ final class Class[A] private (data: ScalaJSClassData[A]) extends Object {
     data.isInstance(obj)
 
   def isAssignableFrom(that: Class[_]): scala.Boolean =
-    if (this.isPrimitive || that.isPrimitive) this eq that
-    else this.isInstance(that.getFakeInstance())
-
-  private def getFakeInstance(): Object =
-    data.getFakeInstance()
+    this.data.isAssignableFrom(that.data)
 
   def isInterface(): scala.Boolean =
     data.isInterface

tools/scalajsenv.js

@@ -963,26 +963,32 @@ getArrayOf() {
 // java.lang.Class support
 
 //!if outputMode != ECMAScript6
-$TypeData.prototype["getFakeInstance"] = function() {
+$TypeData.prototype["isAssignableFrom"] = function(that) {
 //!else
-"getFakeInstance"() {
+"isAssignableFrom"(that) {
 //!endif
-  if (this === $d_T)
-    return "some string";
-  else if (this === $d_jl_Boolean)
-    return false;
-  else if (this === $d_jl_Byte ||
-           this === $d_jl_Short ||
-           this === $d_jl_Integer ||
-           this === $d_jl_Float ||
-           this === $d_jl_Double)
-    return 0;
-  else if (this === $d_jl_Long)
-    return $L0;
-  else if (this === $d_sr_BoxedUnit)
-    return void 0;
-  else
-    return {$classData: this};
+  if (this["isPrimitive"] || that["isPrimitive"]) {
+    return this === that;
+  } else {
+    let thatFakeInstance;
+    if (that === $d_T)
+      thatFakeInstance = "some string";
+    else if (that === $d_jl_Boolean)
+      thatFakeInstance = false;
+    else if (that === $d_jl_Byte ||
+             that === $d_jl_Short ||
+             that === $d_jl_Integer ||
+             that === $d_jl_Float ||
+             that === $d_jl_Double)
+      thatFakeInstance = 0;
+    else if (that === $d_jl_Long)
+      thatFakeInstance = $L0;
+    else if (that === $d_sr_BoxedUnit)
+      thatFakeInstance = void 0;
+    else
+      thatFakeInstance = {$classData: that};
+    return this["isInstance"](thatFakeInstance);
+  }
 };
 
 //!if outputMode != ECMAScript6